This invention is directed to the threaded fastener arts and more particularly to a novel and improved method and apparatus for gaging threaded fasteners and/or fastener blanks.
Such gaging is preferably carried out in order to determine deviations of the dimensions of such fasteners and/or blanks from the specified or desired dimensions, which data may be utilized for statistical in-process control, for lot control and the like. While the method and apparatus of the invention may be utilized with both round body and poly-arcuate or lobular bodied fasteners and their blanks, as well as other similar bodies such as round or poly-arcuate pins, shafts, shanks or the like, the ensuing discussion will be directed primarily to the gaging of poly-arcuate or lobular fasteners. The use of the term "fastener" herein should be understood as including both a fastener blank and a threaded fastener fabricated from such a blank.
The gaging of lobular fasteners has traditionally been a somewhat complicated task, because the degree of lobulation or out-of-roundness is just as important as the basic thread dimensions in the formation of these fasteners. The degree of lobulation, or out of round, is usually defined as half the diametral difference between the inscribed and circumscribed circles of a cross-section arcuate form of the fastener. Such lobular fasteners are used extensively both as thread-forming screws and for self-locking, or sealing purposes in pre-tapped holes. In either of these general applications control of the degree of lobulation is important in assuring proper performance of the product. For example, this degree of lobulation (commonly designated K) must exceed a certain minimum for thread-forming with a reasonable driving effort or for entry into a preformed thread and adequate sealing or self-locking with acceptable driving effort. On the other hand, if this K or degree of lobulation exceeds a certain maximum amount, the tensile load-carrying ability of the joint in the work may be seriously compromised, due to reduced surface thread contact area of the threads.
In the past, conventional screw threads have often been measured by threading each fastener to be tested into each of a pair of so-called ring or functional gages, which have internal threads corresponding with maximum and minimum allowable external thread dimensions of the fastener. An acceptable product was considered to be one which could be threaded into the maximum gage but not into the minimum gage. However, it should be recognized that non-entry into the minimum gage could be the result of only a single over-size thread element or portion, and would not imply any control whatever on the individual minimum thread dimensions of the fastener. In recent years this condition has been alleviated somewhat through the use of pitch diameter micrometers, individual element indicator gages and other techniques.
Past methods of gaging the cross-sectional dimensions of lobular thread-forming and self-locking screws used hand micrometers. In a first measurement, a multi-anvil type micrometer was used, in which the fastener was rotated to obtain the maximum reading; that is, the diameter of the circumscribing circle "C". In a second operation, a more or less conventional micrometer was used to obtain the "D" dimension, which is a cross-section from a high point of one lobe to a low point opposite. It was not the practice, however, to calculate the difference between these two micrometer readings; i.e., the K, or out of round dimension. That is, the dimensions C and D of the fastener alone, even if themselves within acceptable limits, do not guarantee an acceptable K or out of round dimension. Moreover, the two-step inspection process utilizing two micrometers is quite cumbersome to perform, is time-consuming, requiring individual handling and inspection of each part to be inspected and gaged in this method, and also effectively doubles the potential for measurement error.